1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of Related Art
To maintain the image quality of image forming apparatuses, foreign matter such as toner attached to the surface of an intermediate transfer belt has been removed by using a blade that abuts the intermediate transfer belt.
Since such a blade directly abuts the intermediate transfer belt, there is an image forming apparatus that maintains the cleaning performance at a high level by changing the abutting angle of the blade according to the environmental conditions of the image forming apparatus (e.g. the duration of cleaning, the printing mode such as color printing or monochrome printing or the like) (see JP 2014-134620A and JP 2008-003123A).
Since normal blades using a rubber material generate a large frictional force against the intermediate transfer belt, a method of reducing the frictional force against the intermediate transfer belt has been invented which uses a blade abutting member (hereinafter referred to as a rigid blade) having a portion to abut the intermediate transfer belt made of a non-elastic material such as metal that is less elastically deformable than rubber.
However, in such methods that uses a rigid blade for the portion to abut the intermediate transfer belt, the frictional force against the intermediate transfer belt increases in a high-temperature high-humidity environment. In such cases, the rigid blade is pushed in the rotational direction of the intermediate transfer belt by the action of the increased frictional force and thus moves or deforms in the direction of increasing the abutting angle. In contrast, the frictional force against the intermediate transfer belt decreases in a low-temperature low-humidity environment. In such cases, the rigid blade returns in the direction opposite the rotational direction of the intermediate transfer belt to balance the decreased frictional force and thus moves or deforms in the direction of decreasing the abutting angle.
When the environmental condition changes from a high-temperature high-humidity environment to a low-temperature low-humidity environment after the abutting portion of the rigid blade is worn out in a high-temperature and high-humidity environment (in a condition of a large abutting angle), a phenomenon (hereinafter referred to as edge separation) that the worn part is separated from the intermediate transfer belt occurs since the frictional force against the intermediate transfer belt decreases so that the rigid blade moves or deforms in the direction of decreasing the abutting angle.
When such edge separation occurs, toner penetrates into the gap between the intermediate transfer belt and the worn part and acts to lift up the rigid blade. As a result, the rigid blade cannot sufficiently remove foreign matter such as toner attached to the surface of the intermediate transfer belt, and the apparatus eventually cannot maintain the cleaning performance.
However, in the other methods in which the abutting portion elastically deforms, e.g. in cleaning methods using a rubber blade, since the rubber blades has a large coefficient of friction, an external additive that is dissociated from toner is held by the action of the frictional force against the rubber blade and forms an aggregation called a static layer on the upstream side of the abutting portion between the belt and the rubber blade. Therefore, even when a condition similar to edge separation is caused by a change of the environmental condition after a worn part is formed, the static layer of the external additive held at the part fills the gap and prevents the toner from penetrating into the gap between the intermediate transfer belt and the worn part in cooperation with elastic deformation of the rubber blade itself. Therefore, rubber blades do not have such a problem that rigid blades have.
However, the image forming apparatus described in JP 2014-134620A changes the abutting angle or the like according to the duration of abutting time of the blade with the intermediate transfer belt but does not control the abutting angle according to a change of the environmental condition. Therefore, when a rigid blade is employed, edge separation may occur. The image forming apparatus described in JP 2008-003123A is intended to use a rubber blade but is not intended to use a rigid blade in the first place.
Further, the image forming apparatus described in JP 2008-003123A is intended to cope with a change of the trajectory of the intermediate transfer belt between color printing and monochrome printing and does not control the abutting angle in response to a change of the environmental condition. Therefore, when a rigid blade is employed, edge separation may occur.